The administration of the primary bile acid, chenodeoxycholic acid (3 alpha, 7 alpha dihydroxycholanoic acid) to man produces striking changes in biliary bile acid composition. Cholic acid is virtually eliminated from the bile and a new bile acid, ursodeoxycholic acid (3 alpha, 7 beta-dihydroxycholanoic acid), the 7 beta epimer of chenodeoxycholic acid, appears. Since chenodeoxycholic acid is expected to be used widely as an agent to dissolve cholesterol gallstones, increased amounts of ursodeoxycholic acid will circulate in the bile of these subjects. This investigation aims to explore the metabolism of ursodeoxycholic acid in subjects treated with chenodeoxycholic acid and to define (a) the biosynthetic pathway, (b) pool size and daily production rate, (c) tissue or site of formation, (d) biologic significance, and (e) safety. Because of the structural similarity of ursodeoxycholic acid and chenodeoxycholic acid, differing only in the orientation of the hydroxyl group at C-7, key differences in the metabolism of both bile acids will be sought, i.e. turnover rate, intestinal absorption and bacterial dehydroxylation of the C-7 hydroxy group.